NANO EXPRESS

In Situ Generation of Two-Dimensional Au–Pt Core–ShellNanoparticle Assemblies

Madiha Khalid • Natalie Wasio • Thomas Chase •

Krisanu Bandyopadhyay

Received: 8 July 2009 / Accepted: 24 September 2009 / Published online: 14 October 2009

� to the authors 2009

Abstract Two-dimensional assemblies of Au–Pt bime-

tallic nanoparticles are generated in situ on polyethyle-

neimmine (PEI) silane functionalized silicon and indium

tin oxide (ITO) coated glass surfaces. Atomic force

microscopy (AFM), UV–Visible spectroscopy, and elec-

trochemical measurements reveal the formation of core–

shell structure with Au as core and Pt as shell. The core–

shell structure is further supported by comparing with the

corresponding data of Au nanoparticle assemblies. Static

contact angle measurements with water show an increase in

hydrophilic character due to bimetallic nanoparticle gen-

eration on different surfaces. It is further observed that

these Au–Pt core–shell bimetallic nanoparticle assemblies

are catalytically active towards methanol electro-oxidation,

which is the key reaction for direct methanol fuel cells

(DMFCs).

Keywords Au–Pt � Bimetallic � Core–shell �Nanoparticle � Cyclic voltammetry �Atomic force microscopy

Introduction

Nanoparticle assemblies have gained significant attention

recently with the intention of comprehending the true

potential applications of their unique physical, optical, and

electronic properties [1]. The ultimate aim is to interface

these assemblies to microscale and subsequently to mac-

roscale by organizing them into higher-level structures,

devices, and systems with well-defined functionality.

Gold–platinum (Au–Pt) bimetallic nanoparticles as alloy

[2] or core–shell structure [3], in particular, has attracted

increased interest due to its superior performance as fuel

cell catalyst over conventional platinum (Pt) based catalyst

[4–6]. The major problem for Pt-based catalyst is their

poisoning by CO-like intermediates [7, 8]. The unexpected

finding of catalytic activity of gold at the nanoscale [9] has

opened up various new possibilities of catalyst develop-

ment and it is well-known today that presence of Au in

AuPt system enhances the catalytic activity for electro-

chemical methanol oxidation reaction (MOR) as a result of

electronic interaction between Au and Pt or from the lattice

parameter contraction [4]. Currently, these catalysts are

mostly synthesized through direct deposition of Pt on the

preformed gold nanoparticle (Au NP) seeds in solution [3,

10–13] or on solid supports [14, 15]. However, problem

arises from their tendency of coagulation, which occurs due

to the unusual high surface energy. While different capping

agents such as thiols, amines, phosphines, polymers etc. are

normally used in synthesis methods to stabilize and dis-

perse these nanoparticles, interaction with these stabilizing

agents may profoundly alter the catalytic properties of

these systems [5].

Although assemblies of monometallic nanoparticles are

reported extensively [16–19], alloy or core–shell bimetallic

nanoparticle has received very limited attention as building

blocks until recently. Therefore, new methods of assem-

bling these bimetallic nanoparticles on appropriate surfaces

are necessary to integrate them as possible components for

future nanodevices or as novel catalysts for fuel cell

applications. Early attempt of generating assembly of Au–

Pt core–shell nanoparticle (Au–Pt NP) employs the

M. Khalid � N. Wasio � T. Chase � K. Bandyopadhyay (&)

Department of Natural Sciences, University of Michigan—

Dearborn, 4901 Evergreen Road, Dearborn, MI 48128, USA

e-mail: krisanu@umd.umich.edu

123

Nanoscale Res Lett (2010) 5:61–67

DOI 10.1007/s11671-009-9443-2

deposition of Pt layer onto preformed self-assembled seed

Au NPs on silicon surfaces through the reduction of

PtCl62- in presence of NH2OH as the mild reducing agent

[14]. In another recent approach, Au NP film at the air–

water interface was transferred to a solid surface to build a

three dimensional nanoporous structure and finally coated

with a Pt layer to form the desired core–shell structure [15].

Apart from the limited methods available for assembling

core–shell nanoparticles, stabilization against coagulation

and fabrication over a large surface area still remains a

challenge. As an alternative strategy, nanoparticle assem-

blies can be generated in situ inside a suitable template on a

solid surface to avoid number of sequential steps involved

in the methods discussed before. Moreover, the template

will essentially act as a reaction chamber that provides

scaffold for immobilization of specific metal ions, prevent

aggregation, and further act as capping agent to control the

growth of the desired nanoparticle structure. It is also

possible to create lithographically defined structures of a

suitable template to generate nanoparticle patterns without

the multiple steps of synthesis and absorption of the cor-

responding nanoparticles from solution.

In this paper, we report the in situ synthesis of bimetallic

core–shell Au–Pt NP assemblies on silicon and indium tin

oxide (ITO) coated glass surfaces and also demonstrate that

these nanoparticle assemblies are catalytically active

towards methanol electro-oxidation which is one of the key

electrode reactions in direct methanol fuel cells (DMFCs).

Experimental

Materials

Water used in these experiments was purified though a

Millipore system with a resistivity of 18 MX cm. Boron

doped p-type silicon wafers polished on one side (resis-

tivity 10–30 ohm cm) were purchased from Virginia

Semiconductor (Fredericksburg, VA). Indium tin oxide

(ITO) coated glass substrates were obtained from Delta

Technologies, Ltd. (Stillwater, MN) with a resistance of 4–

8 X. Gold (III) chloride trihydrate (HAuCl4�3H2O), chlo-

roplatinic acid hexahydrate (H2PtCl6�6H2O), silver nitrate,

200 proof (absolute) ethanol, absolute methanol, and

sodium citrate tribasic dihydrate were purchased from

Sigma–Aldrich (St. Louis, MO) and were used as received.

Trimethoxysilylpropyl modified polyethylenimine (TSPEI)

as 50% solution in isopropanol was obtained from Gelest,

Inc. (Morrisville, PA) and used without further purification.

Concentrated sulfuric acid, 70% nitric acid, and concen-

trated hydrochloric acids were obtained from Fisher sci-

entific (Pittsburgh, PA).

Surface Functionalization and Nanoparticle Generation

Silicon and ITO surfaces were cleaned before proceeding

to surface modification with TSPEI and subsequent nano-

particle formation. Silicon surfaces, cut into required sizes

for different characterization, were placed in aqua regia

solution (3:1 v/v HCl:HNO3) (Caution! Aqua regia is a

strong oxidizing agent and should be handled with extreme

care) for at least 4 h, rinsed with Millipore water, dried

under a stream of argon, and then heated briefly on a hot

plate to evaporate any residual water. ITO surfaces were

sequentially rinsed in acetone, ethanol, and water, followed

by 1.0 M HCl for 10 min and then treated with 1:1:5 (v/v)

H2O2:NH4OH:H2O for an hour. Finally, these surfaces

were thoroughly rinsed with Millipore water and dried

under flow of argon.

For surface modification, the cleaned samples were

immersed in a 2% solution (v/v) of TSPEI in 95% ethanol

for 6 min [20]. Surfaces were then rinsed with absolute

ethanol, dried under a flow of argon, and left overnight for

curing of the silane layer. In a subsequent step, [AuCl4]-

ion adsorption was done by exposing the surfaces in a

1 9 10-2 M solution of HAuCl4�3H2O for 8 h followed by

rinsing with water and finally drying the surfaces under a

flow of argon. In situ reduction to generate Au NPs on the

surface was achieved by exposing the above surfaces to a

freshly prepared 1% (w/v) aqueous sodium citrate solution

for an additional 8 h. For Au–Pt bimetallic core–shell NP

generation, 1 9 10-2 M HAuCl4 and 1 9 10-2 M H2PtCl6solutions were made separately and mixed in different

volumes to create the desired Au:Pt mole ratio in the final

solution, ranging from 1:0,1:0.25, 1:0.75 to 1:1. After

incubating the surface for 8 h, rinsing with water and

drying under flow of argon, surfaces with adsorbed ions

were placed in a freshly prepared 1% (w/v) aqueous

solution of sodium citrate for 8 h to generate the Au–Pt

NPs.

Characterization of the Nanoparticle Assemblies

Contact angle was measured by Pocket Goniometer, PG-1

model (Paul N. Gardner Company Inc., Pompano Beach,

FL), which is a battery-operated instrument for manual

measurements of static contact angles at ‘‘equilibrium.’’

The plunger was filled with Millipore water, which was

slowly pressed out until it formed a droplet on the surface

(1.5 cm wide 9 2.5–3.0 cm long). At least five different

spots were chosen for measuring the contact angle for each

surface and an average of the values was reported.

UV–visible spectra of gold nanoparticles on ITO sur-

faces were collected using USB4000-UV-VIS spectropho-

tometer from Ocean Optics (Dunedin, FL). Surfaces with

nanoparticles were directly introduced into the cuvette

62 Nanoscale Res Lett (2010) 5:61–67

123

holder (1 cm path length) and spectra were recorded with

corresponding clean unmodified surface as the reference.

Atomic force microscopy (AFM) imaging was per-

formed using Veeco (Santa Barbara, CA) Multimode sys-

tem, equipped with a Nanoscope IIIa controller, in tapping

mode. Cantilevers were phosphorous-doped silicon specific

for tapping mode imaging. The local root-mean-square

(RMS) surface roughness was determined using height data

from at least four representative 2 lm 9 2 lm scan areas

through roughness analysis program included in the AFM

analysis software. Surface coverage of different nanopar-

ticles was estimated by analyzing the mean grain area and

number of grains present in a typical 2 lm 9 2 lm (total

area = 4 lm2) AFM image of the respective nanoparticle.

Cyclic voltammetric (CV) measurements were per-

formed in a Teflon� electrochemical cell (3 ml maximum

volume) using standard three-electrode configuration,

which was controlled by a CHI660C electrochemical

workstation (CH Instruments, Inc. Austin, TX). A coiled

platinum wire was used as the counter electrode, aqueous

Ag/AgCl was used as the reference electrode, and ITO

surfaces were used as the working electrodes.

Results and Discussion

Scheme 1 shows the different steps involved in the gen-

eration of bimetallic core–shell Au–Pt NPs by simulta-

neous in situ reduction of [AuCl4]- and [PtCl6]2- ions

bound to the TSPEI functionalized surface. The multiple

amine functionalities present at the polyethyleneimine

(PEI) backbone of TSPEI can entrap both [AuCl4]- and

[PtCl6]2- ions from solution through electrostatic interac-

tion at a lower pH. The surface functionalization with

TSPEI is achieved through the well-known silane coupling

chemistry of the trimethoxysilane groups present at one

end of the molecule. Two different kinds of surfaces are

used in our experiments with an idea that atomically

smooth silicon is ideal for structural characterization of the

surface bound nanoparticles by AFM, while the conducting

ITO surfaces are suitable to assess the optical and electro-

catalytic activity of the formed nanostructures. Figure 1a

shows a representative AFM image of nearly uniform

spherical Au–Pt NPs on silicon surface after the final

reduction step with the Au:Pt mole ratio of 1:1 in the final

solution. A larger scan size of 5 lm 9 5 lm in Fig. 1b

illustrates the formation of Au–Pt NP assembly over a

larger area, without much long range ordering. An average

height of 7.4 ± 1.3 nm is obtained for the Au–Pt NPs from

the analysis of a number of AFM images of different

samples which is evident from the histogram in the inset of

Fig. 1a. Comparison of monometallic Au NPs generated by

the same procedure shows an average height of

6.3 ± 1.2 nm with a more densely packed structure

(Fig. 1c, inset). It is known that colloidal metal nanopar-

ticles in solution are generated through consecutive steps of

nucleation and growth. The balance between the rate of

nucleation and growth can affect the final particle size. It is

observed that fast nucleation step leads to smaller particles

and slow nucleation results in larger particles. The growth

step can occur mainly by consuming molecular precursors

from the surrounding solution or by Ostwald ripening when

large particles grow at the expense of dissolving a smaller

one. For colloidal metal nanoparticle growth in solution,

Ostwald ripening is mostly absent and the growth usually

happens due to consumption of dissolved metal precursors

from solution. In addition, the fast nucleation event and

following growth step must be completely separate in order

to achieve narrow size distribution of the final nanoparti-

cles while multiple nucleation events may lead to wide size

distribution [21, 22]. The current situation of in situ

nanoparticle generation is different from solution synthesis

since the molecular precursor ([AuCl4]-) is attached to the

template on the surface and no free precursor is essentially

present during nucleation and growth step (during citrate

reduction). The observed Gaussian distribution of particle

size for both Au and Au–Pt systems indicates a single and

fast nucleation event followed by the growth step through

consumption of the surface bound molecular precursor.

The increase in size during bimetallic NP formation com-

pared to its monometallic constituent has been reported in

the literature in the context of core–shell structure forma-

tion in solution [3, 11] and the increase is expected from

the relation [13]

Dcore@shell ¼ Dcore 1þ VshellCshell

VcoreCcore

� �1=3

ð1Þ

where V is the corresponding mole volumes, C is the

overall concentration of the specific metal involved, and D

is the diameter. Alternatively, rather densely packed

assembly generated for Au NPs compared to Au–Pt NPs is

possibly due to the reduction in the number of surface

adsorbed ions during bimetallic NP formation, considering

the difference in ionic charges of the respective ions and

electrostatic interactions working in the process.

Water contact angle measurements are used to follow

the change in surface character during different steps of

Au–Pt NP generation on the surface. Figure 1d shows the

overall trend in the contact angle change from bare silicon

surface to the nanoparticle formation on the surface for

Au:Pt mole ratio of 1:1 in the final solution. Contact angle

increased from 25 ± 2.1� for bare silicon to 54 ± 0.5�with adsorbed [AuCl4]- and [PtCl6]2- on the surface and

eventually decreased to 34 ± 0.57� due to formation of

Au–Pt NPs after the reduction step. This indeed implies the

Nanoscale Res Lett (2010) 5:61–67 63

123

TSPEI

++ ++ ++ ++ ++ ++

++ ++≡≡

≡≡

++ ++++ ++++++ ++++

Si/ITO

HAuCl4+

H2PtCl6

[AuCl4]- ≡

Na-Citrate

++ ++++++ ++++ ++ ++++++ ++++

++ ++++++ ++++≡≡ N

Nn

4n

HH

Si(OCH3)3

Cl

[PtCl6]2- ≡

≡≡≡≡Au-Pt Nanoparticle

Scheme 1 Steps involved in generating Au–Pt bimetallic core–shell nanoparticles on silicon or ITO surfaces. Structure of trimethoxysilylpropyl

modified polyethylenimine (TSPEI) is also shown

(d)

(a) (b)

3 4 5 6 7 8 9 100

4

8

12

16

20

Fre

quen

cy (

%)

Au-nanoparticle height / (nm)3 4 5 6 7 8 9 10

0

4

8

12

16

20

0 10 20 30 40 50 60

Au-Pt Nanoparticle

Monolayer +[AuCl4]-+ [PtCl6]2-

TSPEI Monolayer

Bare Si

Contact angle / degrees

(c)

7.4

4 5 6 7 8 9 10 11 120

2

4

6

8

10

Fre

qu

ency

(%

)

Au-Pt nanoparticle height / (nm)

7.4±1.3 nm

4 5 6 7 8 9 10 11 120

2

4

6

8

10

Fre

qu

ency

(%

)

6.3±1.2 nm

Fig. 1 a 2lm 9 2lm and b 5lm 9 5lm tapping mode AFM height

image of in situ generated bimetallic Au–Pt NPs (Au:Pt mole

ratio = 1:1) on TSPEI modified surface. c 2lm 9 2lm AFM height

image of monometallic Au NPs generated on TSPEI modified surface.

Inset of (a) and (c) show the respective histogram of the Au–Pt and

Au nanoparticle height distribution with a fit (solid line) using

Gaussian distribution function after analyzing a number of AFM

images. Respective mean height and standard deviation are also

shown. d Change in static water contact angle at different steps of

Au–Pt NP formation

64 Nanoscale Res Lett (2010) 5:61–67

123

enhanced hydrophilic character of the surface due to

nanoparticle formation. A similar trend in change of

hydrophilic/hydrophobic character has been observed for

Au NP formation on the surface by the present method and

also reported in the literature for Ag NP formation [23, 24].

Controlling the hydrophilic and hydrophobic properties of

a surface is significant due to different potential application

areas like self-cleaning surfaces and sensors.

The earlier discussion of AFM results points to the

possible core–shell structure formation during in situ syn-

thesis of Au–Pt NPs on the surface from the observed

increase in nanoparticle size going from pure Au NPs to

Au–Pt NPs. However, it is not obvious which metal con-

stitute the core and which one the shell and further

experimental evidence is required to elucidate the actual

structure of these bimetallic Au–Pt NPs generated on the

surface. UV–Visible spectroscopy has proven to be a ver-

satile technique to understand the structure of core–shell

Au–Pt NPs generated in solution and the results are well

documented in the literature [3, 10, 25]. Hence, we syn-

thesized Au NPs and Au–Pt NPs on transparent ITO sur-

faces to assess their optical property, using the same

methodology as discussed above. Figure 2 shows the

comparison of the UV–Visible response of pure Au NPs to

that of Au–Pt NPs with different mole ratio of Au and Pt. It

is evident that Au NPs show a characteristic plasmon

absorption band at 558 nm. Interestingly, the Au surface

plasmon peak for Au–Pt NPs shifted to lower wavelength

(at 548 nm) for a mole ratio of Au:Pt = 1:0.25 and further

blue shifted (a broad peak centered at 538 nm) with

increased Pt content at a ratio of Au:Pt = 1: 0.75. Finally,

the surface plasmon peak for Au completely disappears for

Au:Pt = 1:1. These results, along with several earlier

reports of Au–Pt NP formation in solution [3, 5, 10, 25–27]

substantiate that the deposition of a Pt shell on top of an Au

core is responsible for the disappearance of the Au surface

plasmon peak in the UV–Visible absorption spectrum. In

order to confirm the presence of the Pt shell, Au–Pt

nanoparticles (Au:Pt = 1:1) are generated on quartz sur-

face (transparent to UV) which shows a surface plasmon

peak *222 nm (Fig. 2b) corresponding to zero-valent

platinum [28]. The present results indeed demonstrate the

formation of Au–Pt core–shell NPs on the solid surface and

are particularly significant since the core and the shell are

generated in situ unlike the previous reports in solution

where core particles were initially synthesized and the shell

was deposited subsequently.

In order to further confirm the core–shell structure of

these Au–Pt NPs, electrochemical measurements are done

with Au–Pt NP assemblies generated on ITO surfaces in

aqueous KOH solution and compared to Au NP assemblies

generated on ITO surfaces. Detection of gold oxide (AuOx)

from oxidation/reduction waves in basic (0.5 M KOH)

medium during cyclic voltammetry measurements can

provide information about the chemical nature of the shell.

Figure 3a shows superimposed cyclic voltammograms for

Au NPs and Au–Pt NPs generated on ITO surface in 0.5 M

KOH solution. The presence of oxidation/reduction waves

of AuOx at *0.24 V for Au NPs, in contrast to the absence

of such peak for Au–Pt NPs suggests that the oxidation/

reduction of Au is suppressed by the Pt shell [3] in the

latter, which again validate the core–shell structure of the

present in situ generated Au–Pt NPs. To explore the cata-

lytic properties of these Au–Pt NP bound ITO surfaces

towards methanol oxidation reaction (MOR), cyclic vol-

tammetric responses (Fig. 3b) are recorded in 0.5 M KOH

in presence and absence of MeOH. A strong anodic peak at

*?0.65 V (relative to aqueous Ag/AgCl reference elec-

trode) is observed, corresponding to methanol electro

oxidation [29]. However, this anodic peak disappears in

400 450 500 550 600 650 700 750 8000.00

0.01

0.02

0.03

0.04

0.05

0.06Au:Pt

1:1

1:0.25

1:0

1:0.75

Ab

sorb

ance

(a.

u)

Wavelength / (nm)

200 300 400 500 600 700 8000.00

0.02

0.04

0.06

0.08

0.10

Ab

sorb

ance

(a.

u)

Wavelength / (nm)

Au:Pt =1:1

222nm

(a)

(b)

Fig. 2 a UV–Visible spectrum for Au–Pt bimetallic NPs generated

on ITO surface with varying mole ratio of Au:Pt. Corresponding

Au:Pt ratios are shown on the individual spectrum. b UV–Visible

spectrum for Au–Pt bimetallic NPs generated on quartz surface with

1:1 mole ratio of Au:Pt

Nanoscale Res Lett (2010) 5:61–67 65

123

absence of methanol in solution. It is to be noted that these

Au–Pt NP-bound ITO surfaces were not thermally acti-

vated before assessing their catalytic property.

Generation of Au–Pt core–shell structure from simulta-

neous reduction of surface bound [AuCl4]- and [PtCl6]2-

ions, evident from the UV–Visible and electrochemical

results presented above and the preferred elemental distri-

bution of the core and shell materials warrant further dis-

cussion. Since the formation of nanoparticles in solution

essentially proceeds through nucleation and growth steps, it

is expected that the metal ion which is easier to reduce will

nucleate first and serve as the nucleation site for the second

one during simultaneous reduction of the two metal ions in

solution. In this case, platinum will have to be first reduced

to Pt2? from Pt4? and then to Pt0 with a standard redox

potential of 0.775 V for [PtCl6]2-/[PtCl4]2- and 0.68 V for

[PtCl4]2-/Pt0 compared to a single step reduction for gold

from Au3? to Au0 with a standard reduction potential of

1.002 V for [AuCl4]-/Au0, reported at room temperature

[25]. Considering the reduction potentials, it is obvious that

Au will preferably nucleate first to form the core followed

by Pt to form the shell. However, the situation for surface

bound ions is rather different as the respective ions are

pinned down to the surface through electrostatic attraction

of the template and will have limited mobility. The

underlying mechanism of in situ core–shell structure

formation on the surface is a subject of our on-going

investigation.

Conclusions

In summary, we have reported an elegant method for in situ

generation of two-dimensional Au–Pt bimetallic nanoparti-

cle assemblies on solid surfaces functionalized with poly-

ethyleneimine template. AFM results reveal the formation of

Au–Pt NP assemblies on silicon surface without much long

range ordering and also show an increase in size of these

bimetallic nanoparticles compared to their monometallic Au

equivalents. Comparison of UV–Visible and electrochemi-

cal response of Au–Pt NPs to that of Au NPs generated on

ITO surfaces authenticate the core–shell structure of these

bimetallic nanoparticles. Moreover, these Au–Pt NP

assemblies are active towards methanol oxidation, demon-

strating their potential as catalyst for DMFCs. This in situ

synthetic approach relies on self-assembly employing wet

chemical technique at an ambient condition and can also be

extended to create other bimetallic NP assemblies. Further, it

offers a flexible method to generate bimetallic core–shell

NPs for site selective deposition, nanoparticle patterning for

nanoelectronic applications and can also be combined with

the conventional lithographic techniques.

Acknowledgments We thank the American Chemical Society,

Petroleum Research Fund (ACS-PRF) and National Science Foun-

dation (NSF) for financial support. Office of the Vice President for

Research (OVPR), UM-Ann Arbor and the Office of Research and

Sponsored Programs, UM-Dearborn are also gratefully acknowledged

for additional funding.

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